Low temperature densification of zirconia ceramics

ABSTRACT

Addition of from about 1.0 to about 15% by weight of a vanadium oxide or boric oxide promoter to a zirconia (ZrO 2 ) powder promotes rapid densification at temperatures below about 1300° C. For example, a density of greater than 92% of estimated theoretical density can be achieved by addition of 4% by weight of ZrV 2  O 7  to monoclinic ZrO 2  and sintering at 1100° C. for about 24 hours. Sintering at temperatures below about 1150° C. avoids the destructive monoclinic/tetragonal phase transformation and provides for better control of microstructure and composition. A density of greater than 98% of estimated theoretical density can be achieved by addition of from about 2 to about 5% by weight of a boric oxide promoter to stabilized cubic ZrO 2  and sintering at about 1200° C. for about 4 hours.

This invention was made in the course of work supported by a grant fromthe Office of Naval Research, Department of the Navy.

This invention relates to densified zirconia (ZrO₂) ceramic compositionsand to processes for densifying such compositions. In one aspect, thisinvention involves the low temperature densification of monoclinic,unstabilized ZrO₂ with the aid of a vanadium oxide promoter. In anotheraspect, this invention involves the low temperature densification ofcubic, stabilized ZrO₂ with the aid of a boric oxide promoter.

Densified ZrO₂ ceramic compositions are well known and widely used asrefractory materials for high temperature furnace applications. Inaddition, because of the high ionic conduction of the oxide, ZrO₂ceramics are being used increasingly in electronic applications, forexample as solid electrolyte for fuel cell and oxygen sensorapplications. Densification of the oxide is conventionally carried outat temperatures from about 1700° to about 1900° C. However, attemperatures of about 1150° C., unstabilized ZrO₂ undergoes amonoclinic/tetragonal phase transformation with a destructive volumecontraction of about 9%. To prevent this destructive phasetransformation, the ZrO₂ can be stabilized by addition of up to about 15mol % of calcium oxide, magnesium oxide, or yttrium oxide. In additionto functioning as stabilizers, the added oxides increase oxygen ionmobility and therefore the conductivity of the ZrO₂ composition. Butwith the high densification temperatures conventionally required, it isdifficult to obtain precise control of the microstructure and impuritylevels in the final composition to the extent necessary for electrolyticapplications.

Accordingly, it is an object of this invention to densify monoclinic,unstabilized ZrO₂ to a density greater than about 90% of theoreticaldensity at a temperature below about 1150° C., thereby avoiding themonoclinic/tetragonal phase transformation.

Another object of this invention is to densify cubic, stabilized ZrO₂ toa density of greater than about 98% of theoretical density at atemperature of about 1200° C.

Still another object is to provide improved control of composition andmicrostructure of the ZrO₂ ceramic composition.

A further object is to provide economies in the processing of ZrO₂ceramics by virtue of lower firing temperatures.

These and other objects will become apparent as description of theinvention proceeds.

It has now been found that low temperature densification of ZrO₂ can beeffected by liquid phase sintering with from about 1.0 to about 15% byweight of a low melting vanadium oxide or boric oxide densificationpromoter. Suitable promoters are the oxides of vanadium and boron thatmelt below about 1200° C., preferably those oxides that melt congruentlyto provide low viscosity liquids above their melting points.Particularly effective vanadium oxide promoters are ZrV₂ O₇ (m.p. 790°C.) and V₂ O₅ (m.p. 690° C.). Particularly effective boric oxidepromoters are calcium metaborate (CaO.B₂ O₃, m.p. 1156° C.),calciumtetraborate(CaO.2B₂ O₃, m.p. 986° C.), and B₂ O₃ (m.p. 460° C.).

Addition of from about 1 to about 10%, preferably from about 2 to about5%, by weight of an oxide of vanadium to a ZrO₂ powder having amonoclinic crystal configuration promotes rapid densification attemperatures below about 1150° C. Sintering is suitably carried out forup to about 24 hours and densities of greater than 90% of theoreticalare readily obtained. Sintering below 1150° C. avoids themonoclinic/tetragonal phase transformation and eliminates the need foruse of stabilizers such as CaO, MgO, and Y₂ O₃. The absence of addedstabilizers results in better control of impurity levels in thedensified ZrO₂ ceramic composition.

Addition of from about 1 to about 15%, preferably from about 2 to about10%, by weight of an oxide of boron to a ZrO₂ powder stabilized in thecubic crystal configuration provides rapid densification at temperaturesbelow about 1350° C. Densities of greater than 98% of theoretical areobtained by sintering at 1200° C. for up to about 4 hours. The ZrO₂powder can be stabilized in the manner well known in the art, forexample by addition of from about 1 to about 15% by weight of CaO or Y₂O₃. Commercially available ZrO₂ powders, stabilized with 6.5% by weightof CaO or with 12% by weight of Y₂ O₃, can be employed advantageously inthe practice of this invention. Sintering under the above-prescribedconditions results in partial destabilization of cubic to monoclinicZrO₂. Destabilization to provide up to about 30% by weight of themonoclinic crystal configuration is desirable in that it induces a largevolume of microcracks in the ceramic which enhances thermal shockresistance, (Garvie, R. C., et al., J. Am. Ceram. Soc. 55(3) 152-157,1972). Use of the boric oxide promoters of this invention permitsprecise control of the amount of monoclinic ZrO₂ formed, and thus thefracture toughness and thermal shock resistance of the fired ceramic.

The ZrO₂ powders that can be used in this invention are readilyavailable and typically show the properties listed in the followingtable.

                  TABLE I                                                         ______________________________________                                        Zirconia(ZrO.sub.2) Powders.                                                                          Stabilized                                                         Monoclinic ZrO.sub.2                                                                     Cubic ZrO.sub.2                                       ______________________________________                                        M.p., °C.                                                                             2680         2625                                              Particle size  -325 mesh    -325 mesh                                                        ave. 1.5 micron                                                                            ave. 1.5 micron                                   Theoretical density, g/cc                                                                    5.56-5.68    5.56-6.05                                         ZrO.sub.2 (min)                                                                              99%          93.5%                                             SiO.sub.2      0.18         0.62                                              CaO            0.22         4.80                                              MgO            0.15         0.25                                              Fe.sub.2 O.sub.3                                                                             0.10         0.10                                              Al.sub.2 O.sub.3                                                                             0.16         0.18                                              TiO.sub.2      0.11         0.11                                              ______________________________________                                    

The vanadium oxide and boric oxide densification promoters arepreferably of reagent grade purity, and can be employed in the form ofoxides having the formula V₂ O₅ and B₂ O₃ ; derivatives that afford suchoxides during firing can also be used. A particularly useful vanadiumoxide derivative has the formula ZrV₂ O₇ and can be prepared by theprocedure of Peyronel, Gazz. Chim. Ital., 72 77-83 (1942), whichprocedure involves precipitation from a solution containing equimolaramounts of ZrO₂ and V₂ O₅ in the form of ZrO(NO3)₂ and NH₄ VO₃, followedby calcination at 330° C. to drive off ammonium salts. Particularlyuseful boric oxide derivatives are calcium metaborate (CaO.B₂ O₃), andcalcium tetraborate (CaO.2B₂ O₃).

In the densification process, the ZrO₂ powder and the added promoter aremilled and blended, formed and pressed at about 5000 to about 40,000psi, and then fired in air. In the case of monoclinic ZrO₂ with avanadium oxide promoter, the mixture is fired at a temperature of fromabout 1050° to about 1150° C. for from about 0.5 to about 24 hours toprovide densification of greater than about 90% of theoretical value. Inthe case of stabilized, cubic ZrO₂ with a boric oxide promoter, themixture is fired at a temperature of from about 1150° to about 1350° C.for from about 0.5 to about 4 hours to provide densification greaterthan about 98% of theoretical value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the percent of theoretical density obtained as afunction of sintering time for monoclinic ZrO₂ powders promoted withvarious amounts of ZrV₂ O₇.

FIG. 2 illustrates the percent of theoretical density obtained as afunction of the addition of various amounts of boric oxide promoters fora CaO-stabilized, cubic ZrO₂ powder fired at 1200° C. for 4 hours.

FIG. 3 shows the amount of monoclinic ZrO₂ formed from CaO-stabilized,cubic ZrO₂ as a function of added boric oxide promoters with firing at1200° C. for 4 hours.

FIG. 4 shows the amount of monoclinic ZrO₂ formed from Y₂ O₃-stabilized, cubic ZrO₂ as a function of added boric oxide promoterswith firing at 1200° C. for 4 hours.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is further illustrated by reference to the followingprocedures and examples.

Various samples were prepared from mixtures of monoclinic ZrO₂ andvanadium oxide promoters. Amounts of from 1.0 to 10% by weight were usedin the case of ZrV₂ O₇ promoter. Amounts of from 0.25 and 6.5% by weightwere used in the case of V₂ O₅ promoter. The samples, 50 to 100 g insize, were weighed blended and milled in plastic jars containing densezirconia balls. The procedure followed was to wet mill for 2-5 hours inisopropanol, vacuum dry, and then dry mill for an additional 2 hours. Adilute polyvinyl alcohol solution was used as a binder to aid inpressing. The samples were granulated and passed through a 35-meshscreen prior to pressing.

Sample discs about 1.6 cm in diameter and about 0.3 cm thick wereprepared by pressing in a hydraulic press at about 25,000 psi. Thesamples were then placed on platinum sheets and fired at temperatures offrom about 1050° to about 1125° C. for from about 0.5 to about 4 hours.

Bulk density measurements were made on the fired samples with a mercuryhydrometer. The densification data are plotted in FIG. 1 to show thepercent of theoretical density obtained with increasing sintering timefor monoclinic ZrO₂ powders promoted with various amounts of ZrV₂ O₇. Itis seen that at 1100° C. use of 2 to 6% by weight of ZrV₂ O₇ provided ahigh degree of densification with sintering times of 4 to 24 hours.Greater than 92% of theoretical density was obtained with 3 to 4% byweight of ZrV₂ O₇ and a sintering time of 24 hours. Essentiallyequivalent results were obtained with added V₂ O₅ promoter. In contrast,a control composition containing no added promoter showed essentially nodensification under these conditions.

Two stabilized, cubic ZrO₂ powders were used in the following examples:one stabilized with 6.5% by weight of CaO, the other stabilized with 12%by weight of Y₂ O₃. The boric oxide promoters used were reagent grade B₂O₃, calcium metaborate, and calcium tetraborate, the last two made bymelting appropriate amounts of CaCO₃ and B₂ O₃, followed by crushing andmilling in methanol. The promoters were added in amounts of 2, 5, and10% by weight. Samples of ZrO₂ powder and promoter weighing 15 g wereblended and milled in plastic bottles using zirconia grinding balls andmethanol as dispersant. After milling for 4 to 5 hours, the powders werevacuum dried. A dilute methanol solution of carbowax was used as abinder to aid in pressing.

Sample discs about 1.25 cm in diameter and about 0.2 cm thick were madeby pressing in a hydraulic press at 22,000 psi. The samples were placedon platinum sheets and fired at 1200° C. for 4 hours and at 1350° C. for4 hours.

Bulk density measurements on the fired samples were made with a mercuryhydrometer. Table II shows the bulk and theoretical densities forCaO-stabilized ZrO₂ samples promoted with various amounts of calciummetaborate, calcium tetraborate, and B₂ O₃. It is seen that with nopromoter, the fired densities at 1200° and 1350° C. were 85% and 95% oftheoretical density. With 2% calcium metaborate addition, there was nochange in densification at 1200° C., and a decrease at 1350° C. Thisdecrease probably reflects the fact that insufficient liquid was presentto aid densification by particle rearrangement but was probablysufficient to cause relaxation or expansion of the highly compresseddisc. With 5% calcium metaborate addition, densification was 99% and100% of theoretical at 1200° and 1350° C. With 10% calcium metaborateaddition, essentially complete densification was also obtained.

With calcium tetraborate, increased densification was obtained with 2%addition and complete densification with 5% addition. With 10% addition,there was some decrease in densification because of excess of liquidphase and some puddling was also observed.

With B₂ O₃ promoter densification reached a maximum for 2% addition at1200° C. This can be attributed to the fact that B₂ O₃ combines with theCaO stabilizer to form a low melting calcium borate that is liquid above1100° C. Even with 1% B₂ O₃ addition enough liquid was formed to providesignificant densification at 1200° C.

Table III shows bulk and theoretical densities for Y₂ O₃ -stabilizedZrO₂ samples promoted with various amounts of calcium metaborate,calcium tetraborate, and B₂ O₃. It is seen that Y₂ O₃ -stabilized ZrO₂is far less responsive than is CaO-stabilized ZrO₂ to boric oxidepromoters. Additions at the 10% level were needed to obtain fulldensities at 1200° C. The least active promoter was B₂ O₃ which, incontrast, was the most reactive promoter with CaO-stabilized ZrO₂.

                                      TABLE II                                    __________________________________________________________________________    Fired Densities for CaO-Stabilized ZrO.sub.2 Compositions                                        Fired Densities                                                               1200° C.                                                                          1350° C.                                              Theoretical                                                                         Bulk Theoretical                                                                         Bulk Theoretical                                Composition  Density, %                                                                          Density                                                                            Density, %                                                                          Density                                                                            Density, %                                 __________________________________________________________________________    CaO-Stabilized ZrO.sub.2                                                                   5.5   4.6  85    5.2  95                                         +2 wt% Ca metaborate                                                                       5.4   4.6  85    4.9  91                                         +2 wt% Ca tetraborate                                                                      5.4   5.0  92    5.2  97                                         +1 wt% B.sub.2 O.sub.3                                                                     5.4   5.1  95    --   --                                         +5 wt% Ca metaborate                                                                       5.2   5.2  99    5.3  100                                        +5 wt% Ca tetraborate                                                                      5.2   5.2  100   --   --                                         +5 wt% B.sub.2 O.sub.3                                                                     5.2   5.2  100   5.3  100                                        +10 wt% Ca metaborate                                                                      5.0   4.9  99    4.9  99                                         +10 wt% Ca tetraborate                                                                     5.0   5.1  100 4.9                                                                             98                                              +10 wt% B.sub.2 O.sub.3                                                                    5.0   4.9  99    --   --                                         __________________________________________________________________________

                                      TABLE III                                   __________________________________________________________________________    Fired Densities for Y.sub.2 O.sub.3 -Stabilized ZrO.sub.2 Compositions                           Fired Densities                                                               1200° C.                                                                          1350° C.                                              Theoretical                                                                         Bulk Theoretical                                                                         Bulk Theoretical                                Composition  Density, %                                                                          Density                                                                            Density, %                                                                          Density                                                                            Density, %                                 __________________________________________________________________________    Y.sub.2 O.sub.3 -Stabilized ZrO.sub.2                                                      6.1   5.4  89    5.8  95                                         +2 wt% Ca metaborate                                                                       5.9   4.7  80    4.9  82                                         +2 wt% Ca tetraborate 5.9                                                                  4.6   77   5.7   97                                              +2 wt% B.sub.2 O.sub.3                                                                     5.9   4.3  73    --   --                                         +1 wt% B.sub.2 O.sub.3                                                                     5.9   4.6  77    --   --                                         +5 wt% Ca metaborate                                                                       5.7   5.5  96    5.3  93                                         +5 wt% Ca tetraborate                                                                      5.7   5.3  93    5.6  93                                         +5 wt% B.sub.2 O.sub.3                                                                     5.7   4.3  76    --   --                                         +10 wt% Ca metaborate                                                                      5.4   5.3  99    5.3  98                                         °10 wt% Ca tetraborate 5.4                                                          5.4   100  5.3   98                                              +10 wt% B.sub.2 O.sub.3                                                                    5.4   5.2  99    --   --                                         __________________________________________________________________________

X-ray diffraction analysis of the CaO-stabilized and Y₂ O₃ -stabilizedZrO₂ samples fired at 1200° C. with boric oxide promoters showed thepresence of both monoclinic and cubic ZrO₂. In FIG. 3 the percent ofmonoclinic phase present in fired CaO-stabilized samples is plottedagainst the amount of promoter used. The amount of monoclinic ZrO₂formed is seen to increase linearly with the amount of promoter used,with B₂ O₃ being the most reactive. In FIG. 4 the percent of monoclinicZrO₂ present in fired Y₂ O₃ -stabilized samples is plotted against theamount of promoter used. Here it is seen that far less monoclinic phaseis formed which is in keeping with the density data that less reactiontakes place between the Y₂ O₃ -stabilized samples and the boric oxidepromoters.

It is clear that the herein disclosed invention provides highlyeffective promoters for densifying zirconia powders at low temperatures.Monoclinic ZrO₂ can be densified with the aid of vanadium oxidepromoters at temperatures as low as 1100° C. in a process which avoidsthe destructive monoclinic/tetragonal phase transformation. Stabilized,cubic ZrO₂ can be successfully densified with the aid of boric oxidepromoters at temperatures of about 1200° C., well below those requiredwithout added promoter, and with controlled destabilization of cubic tomonoclinic ZrO₂.

Although this invention has been described with particular reference tocertain preferred embodiments thereof, it is understood that variationsand modifications can be effected within the spirit and scope of theappended claim. It is intended that all matter contained in the abovedescription, tables, and figures shall be interpreted in an illustrativeand not in a limiting sense.

What is claimed is:
 1. A fired ceramic composition having a densitygreater than about 98% of theoretical density which composition consistsessentially of a major amount of zirconia and from about 1.0 to about15% by weight of a boric oxide densification promoter wherein the boricoxide promoter is selected from the group consisting of calciummetaborate, calcium tetraborate, and B₂ O₃ ; and wherein the zirconiacomponent consists essentially of from about 5 to about 30% by weight ofzirconia in the monoclinic crystal configuration, and from about 95 toabout 70% by weight of zirconia stabilized in the cubic crystalconfiguration with from about 5 to about 15% by weight of an oxideselected from the group consisting of CaO, MgO, and Y₂ O₃.
 2. Thecomposition of claim 1 wherein the boric oxide promoter is selected fromthe group consisting of calcium metaborate, calcium teraborate, and B₂O₃, said promoter being present in amounts of from about 2 to about 10%by weight; and wherein the zirconia component consists essentially offrom about 10 to about 30% by weight of monoclinic zirconia and fromabout 90 to about 70% by weight of cubic zirconia stabilized with CaO.3. The composition of claim 1 wherein the boric oxide promoter isselected from the group consisting of calcium metaborate, calciumtetraborate, and B₂ O₃, said promoter being present in amounts of about10% by weight; and wherein the zirconia component consists essentiallyof from about 10 to about 15% by weight of monoclinic zirconia and fromabout 90 to about 85% by weight of cubic zirconia stabilized with Y₂ O₃.4. A fired ceramic composition having a density greater than about 90%of theoretical density and being substantially entirely in themonoclinic crystal configuration which composition is prepared by thesteps of milling and blending a zirconia powder substantially entirelyin the monoclinic crystal configuration with from about 1 to about 10%by weight of a vanadium oxide promoter having a melting point belowabout 1200° C.; and pressing and firing the milled and blended powder inair at a temperature of from about 1050° to about 1125° C. for fromabout 0.5 to about 24 hours.
 5. The composition of claim 4 wherein thevanadium oxide promoter is selected from the group consisting of ZrV₂ O₇and V₂ O₅ in amounts of from about 2 to about 6% by weight, and thefiring temperature is from about 1075° to about 1100° C. for from about4 to about 24 hours.
 6. The composition of claim 1 having a density ofgreater than about 98% of theoretical density prepared by the stepscomprising milling and blending a zirconia powder, stabilized in thecubic crystal configuration with from about 5 to about 15% by weight ofan oxide selected from the group consisting of CaO, MgO, Y₂ O₃, withfrom about 1 to about 15% by weight of a boric oxide promoter having amelting point below about 1200° C.; and pressing and firing the milledand blended powder in air at a temperature of from about 1150° to about1350° C. for from about 0.5 to about 4 hours to effect saiddensification and to convert from about 5 to about 30% by weight of thecubic zirconia to monoclinic zirconia.
 7. The composition of claim 6wherein the boric oxide promoter is selected from the group consistingof calcium metaborate, calcium tetraborate, and B₂ O₃ in amounts of fromabout 2 to about 10% by weight, and the firing temperature is from about1200° to about 1350° C. for from about 0.5 to about 4 hours.
 8. Aprocess for preparing a zirconia ceramic composition having a densitygreater than about 90% of theoretical density and being substantiallyentirely in the monoclinic crystal configuration comprising the steps ofmilling and blending a zirconia powder substantially entirely in themonoclinic crystal configuration with about from 1 to about 10% byweight of a vanadium oxide promoter having a melting point below about1200° C.; and pressing and firing the milled and blended powder in airat a temperature of from about 1050° to about 1125° C. for from about0.5 to about 24 hours.
 9. The process of claim 8 wherein the vanadiumoxide promoter is selected from the group consisting of ZrV₂ O₇ and V₂O₅ in amounts of from about 2 to about 6% by weight; and the firingtemperature is from about 1075° to about 1100° C. for from about 4 toabout 24 hours.
 10. A process for preparing a zirconia ceramiccomposition having a density greater than about 98% of theoreticaldensity comprising the steps of milling and blending a zirconia powder,stabilized in the cubic crystal configuration with from about 5 to about15% by weight of an oxide selected from the group consisting of CaO,MgO, and Y₂ O₃, with from about 1 to about 15% by weight of a boricoxide promoter having a melting point below about 1200° C.; and pressingand firing the milled and blended powder in air at a temperature of fromabout 1150° to about 1350° C. for from about 0.5 to about 4 hours toeffect said densification and to convert from about 5 to about 30% byweight of the cubic zirconia to monoclinic zirconia.
 11. The process ofclaim 10 wherein the boric oxide promoter is selected from the groupconsisting of calcium metaborate, calcium tetraborate, and B₂ O₃ inamounts of from about 2 to about 10% by weight, and the firingtemperature is from about 1200° to about 1350° C. for from about 0.5 toabout 4 hours.